Semiconductor device

ABSTRACT

A highly reliable semiconductor device is provided. The semiconductor device includes a gate electrode, a gate insulating film over the gate electrode, a semiconductor film overlapping with the gate electrode with the gate insulating film positioned therebetween, a source electrode and a drain electrode that are in contact with the semiconductor film, and an oxide film over the semiconductor film, the source electrode, and the drain electrode. An end portion of the semiconductor film is spaced from an end portion of the source electrode or the drain electrode in a region overlapping with the semiconductor film in a channel width direction. The semiconductor film and the oxide film each include a metal oxide including In, Ga, and Zn. The oxide film has an atomic ratio where the atomic percent of In is lower than the atomic percent of In in the atomic ratio of the semiconductor film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor devices utilizingsemiconductor characteristics.

2. Description of the Related Art

In recent years, a metal oxide having semiconductor characteristicscalled an oxide semiconductor has attracted attention as a novelsemiconductor material having high mobility provided by crystallinesilicon and uniform element characteristics provided by amorphoussilicon. The metal oxide is used for various applications. For example,indium oxide, which is a well-known metal oxide, is used for alight-transmitting pixel electrode in a liquid crystal display device, alight-emitting device, or the like. Examples of such a metal oxidehaving semiconductor characteristics include tungsten oxide, tin oxide,indium oxide, and zinc oxide. Transistors each including such a metaloxide having semiconductor characteristics in a channel formation regionhave been known (Patent Documents 1 and 2).

REFERENCE

-   Patent Document 1: Japanese Published Patent Application No.    2007-123861-   Patent Document 2: Japanese Published Patent Application No.    2007-096055

SUMMARY OF THE INVENTION

Electrical characteristics (e.g., threshold voltage) of a transistorused in a semiconductor device need to be hardly changed due todeterioration over time. In particular, in a circuit includingtransistors having the same conductivity type, a potential output fromthe circuit is easily influenced by the threshold voltage of thetransistor. Thus, the allowable range of the threshold voltage of thetransistor in the circuit including transistors having the sameconductivity type tends to be narrower than that of a CMOS circuit.Accordingly, in a semiconductor device, especially, a semiconductordevice that includes a circuit including transistors having the sameconductivity type, it is important to use transistors whose electricalcharacteristics are hardly changed due to deterioration over time inensuring reliability.

Desired electrical characteristics of a semiconductor element in asemiconductor device vary depending on circuit design. In the case of ann-channel transistor that needs to be off when gate voltage is lowerthan or equal to 0 V, that is, needs to be normally off, the thresholdvoltage needs to be higher than 0 V. Accordingly, the threshold voltageof the transistor needs to be hardly changed due to deterioration overtime and to have an initial value which makes the transistor be normallyoff.

In view of the above technical background, it is an object of thepresent invention to provide a semiconductor device including a normallyoff transistor. It is another object of the present invention to providea highly reliable semiconductor device.

An initial value of the threshold voltage of a transistor and the amountof change in threshold voltage due to deterioration over time varydepending on relation between the layout of a semiconductor film and thelayout of a conductive film functioning as a source electrode or a drainelectrode. In one embodiment of the present invention, this relation canbe used to achieve the above object.

Specifically, a semiconductor device according to one embodiment of thepresent invention includes a gate electrode, a gate insulating film, asemiconductor film that overlaps with the gate electrode with the gateinsulating film positioned therebetween, and a source electrode and adrain electrode that are in contact with the semiconductor film. An endportion of the semiconductor film is spaced from an end portion of thesource electrode or the drain electrode in a region overlapping with thesemiconductor film in a channel width direction.

When an end portion of a semiconductor film including an oxidesemiconductor is exposed to plasma by etching for forming the endportion, chlorine radical, fluorine radical, or the like generated froman etching gas is easily bonded to a metal element contained in theoxide semiconductor. Thus, in the end portion of the semiconductor film,oxygen bonded to the metal element is easily eliminated, so that anoxygen vacancy is easily formed. However, in one embodiment of thepresent invention, with such a structure, an end portion of thesemiconductor film that overlaps with neither a source electrode nor adrain electrode, that is, an end portion of the semiconductor film in aregion different from the region where the source electrode and thedrain electrode are formed can be made long. Further, in the end portionof the semiconductor film that overlaps with neither the sourceelectrode nor the drain electrode, that is, the end portion of thesemiconductor film in the region different from the region where thesource electrode and the drain electrode are formed, the density of aline of electric force extending from the drain electrode to the sourceelectrode can be lowered to decrease an electric field applied to theend portion. Accordingly, even when an oxygen vacancy is formed in theend portion of the semiconductor film, leakage current flowing betweenthe source electrode and the drain electrode through the end portion canbe reduced when a transistor needs to be turned off. Consequently, thethreshold voltage of the transistor can be controlled so that thetransistor is normally off.

In one embodiment of the present invention, by decreasing an electricfield applied to the end portion of the semiconductor film, it ispossible to prevent an electron (carrier) from being trapped in the gateinsulating film from the end portion. As a result, changes in thresholdvoltage can be suppressed, so that the reliability of the semiconductordevice can be increased.

In one embodiment of the present invention, with such a structure, asemiconductor device including a normally off transistor can beprovided. Further, in one embodiment of the present invention, with sucha structure, a highly reliable semiconductor device can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are a top view and cross-sectional views of a transistor;

FIGS. 2A and 2B are top views of transistors;

FIGS. 3A to 3D are a top view and cross-sectional views of a transistor;

FIG. 4 is a top view of the transistor;

FIGS. 5A and 5B are top views of the transistor;

FIGS. 6A to 6D are a top view and cross-sectional views of a transistor;

FIGS. 7A and 7B are top views of the transistors;

FIG. 8 is a graph showing the measured amount of change in thresholdvoltage and the measured amount of change in shift value;

FIG. 9 is a cross-sectional view of a transistor;

FIGS. 10A to 10D illustrate a method for forming a semiconductor device;

FIGS. 11A to 11C illustrate the method for forming a semiconductordevice;

FIGS. 12A to 12C illustrate structures of a shift register and asequential logic circuit;

FIGS. 13A to 13C illustrate a structure of a semiconductor displaydevice;

FIGS. 14A to 14F illustrate electronic devices;

FIGS. 15A and 15B show band structures of a stack of oxides according toone embodiment of the present invention;

FIGS. 16A and 16B are graphs each showing relation between gate voltageand drain current of a transistor;

FIGS. 17A and 17B are graphs each showing relation between gate voltageand drain current of a transistor; and

FIG. 18 is a cross-sectional view of a transistor.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail belowwith reference to the drawings. Note that the present invention is notlimited to the following description. It will be readily appreciated bythose skilled in the art that modes and details of the present inventioncan be modified in various ways without departing from the spirit andscope of the present invention. The present invention therefore shouldnot be construed as being limited to the following description of theembodiments.

Note that the present invention includes, in its category, all thesemiconductor devices that include transistors: for example, integratedcircuits, RF tags, and semiconductor display devices. The integratedcircuit includes, in its category, large scale integrated circuits(LSIs) including a microprocessor, an image processing circuit, adigital signal processor (DSP), and a microcontroller and programmablelogic devices (PLDs) such as a field programmable gate array (FPGA) anda complex PLD (CPLD). Further, the semiconductor display deviceincludes, in its category, semiconductor display devices in whichtransistors are included in driver circuits, such as liquid crystaldisplay devices, light-emitting devices in which a light-emittingelement typified by an organic light-emitting element is provided ineach pixel, electronic paper, digital micromirror devices (DMDs), plasmadisplay panels (PDPs), and field emission displays (FEDs).

<Aspect 1 of Transistor>

FIGS. 1A to 1C illustrate one aspect of a transistor included in asemiconductor device according to one embodiment of the presentinvention. FIG. 1A is a top view of a transistor 10. FIG. 1B correspondsto a diagram illustrating a cross-sectional structure of the transistor10 in FIG. 1A taken along broken line A1-A2. FIG. 1C corresponds to adiagram illustrating a cross-sectional structure of the transistor 10 inFIG. 1A taken along broken line A3-A4. Note that insulating films suchas a gate insulating film are not illustrated in FIG. 1A in order toclarify the layout of the transistor 10.

As illustrated in FIGS. 1A to 1C, the transistor 10 includes, over asubstrate 11 having an insulating surface, a conductive film 12functioning as a gate electrode, a gate insulating film 13 over theconductive film 12, a semiconductor film 14 overlapping with theconductive film 12 with the gate insulating film 13 positionedtherebetween, and a conductive film 15 and a conductive film 16 that arein contact with the semiconductor film 14 and function as a sourceelectrode and a drain electrode.

In FIGS. 1A to 1C, an oxide film 17 is provided over the semiconductorfilm 14, the conductive film 15, and the conductive film 16. In oneembodiment of the present invention, the oxide film 17 may be acomponent of the transistor 10.

In FIG. 1A, a direction in which a carrier moves between the conductivefilm 15 and the conductive film 16 in the shortest distance is referredto as a channel length direction (indicated by an arrow D1). Inaddition, in FIG. 1A, a direction perpendicular to the channel lengthdirection is referred to as a channel width direction (indicated by anarrow D2).

In one embodiment of the present invention, an end portion of thesemiconductor film 14 is spaced from an end portion of the conductivefilm 15 or the conductive film 16 in a region overlapping with thesemiconductor film 14 in the channel width direction. From anotherperspective, it can be said that in the transistor 10, the width Wi ofthe semiconductor film 14 in the channel width direction is larger thanthe width Wsd of the conductive film 15 or the conductive film 16 in aregion 18 where the conductive film 15 or the conductive film 16overlaps with the semiconductor film 14 in the channel width direction.

Note that in one embodiment of the present invention, end portions ofthe semiconductor film 14 are spaced from end portions of the conductivefilm 15 and the conductive film 16 in the channel width direction in theregion 18. FIG. 1A illustrates an example in which the end portions ofthe semiconductor film 14 and the end portions of the conductive film 15and the conductive film 16 in the region 18 have a space Wd1 and a spaceWd2.

In one embodiment of the present invention, with such a structure, thetransistor 10 can be normally off, and changes in threshold voltage canbe prevented. The reason for this is described in detail below.

Lines of electric force (indicated by broken arrows) are shown betweenthe conductive film 15 and the conductive film 16 in the top view of thetransistor 10 in FIG. 2A. FIG. 2A illustrates lines of electric force atthe time when the transistor 10 is an n-channel transistor, theconductive film 15 is a drain electrode, and the conductive film 16 is asource electrode.

In the transistor 10 in FIG. 2A, the lines of electric force extend fromthe conductive film 15 (drain electrode) to the conductive film 16(source electrode). In the transistor 10, the lines of electric forceexist in a region 19 a including a path through which the conductivefilm 15 is connected to the conductive film 16 in the channel lengthdirection indicated by the arrow D1 in the semiconductor film 14.Further, in the transistor 10, the lines of electric force exist notonly in the region 19 a but also in a region 19 b that is off the pathin the semiconductor film 14 to wrap around the conductive film 15 andthe conductive film 16.

Next, FIG. 2B illustrates a top view of a transistor 20 having astructure different from that of the transistor 10 and lines of electricforce (indicated by broken arrows) as a comparison example.

The transistor 20 includes, over an insulating surface, a conductivefilm 22 functioning as a gate electrode, a gate insulating film (notillustrated) over the conductive film 22, a semiconductor film 24overlapping with the conductive film 22 with the gate insulating filmpositioned therebetween, and a conductive film 25 and a conductive film26 that are in contact with the semiconductor film 24 and function as asource electrode and a drain electrode.

In the transistor 20, an end portion of the conductive film 25 or theconductive film 26 is spaced from an end portion of the semiconductorfilm 24 in a region overlapping with the conductive film 25 or theconductive film 26 in the channel width direction indicated by the arrowD2. From another perspective, it can be said that in the transistor 20,the width Wi of the semiconductor film 24 in the channel width directionis smaller than the width Wsd of the conductive film 25 or theconductive film 26 in the channel width direction.

FIG. 2B illustrates an example in which end portions of the conductivefilm 25 or the conductive film 26 and end portions of the semiconductorfilm 24 in the region overlapping with the conductive film 25 or theconductive film 26 in the channel width direction have a space Wd3 and aspace Wd4.

FIG. 2B illustrates lines of electric force at the time when thetransistor 20 is an n-channel transistor, the conductive film 25 is adrain electrode, and the conductive film 26 is a source electrode.

In the transistor 20 in FIG. 2B, the lines of electric force extend fromthe conductive film 25 (drain electrode) to the conductive film 26(source electrode). In the transistor 20, the lines of electric forceexist only along a path through which the conductive film 25 isconnected to the conductive film 26 in the channel length directionindicated by the arrow D1 in the semiconductor film 24.

Thus, when the end portions of the semiconductor film 14 in FIG. 2A arecompared to the end portions of the semiconductor film 24 in FIG. 2B,end portions of the semiconductor film 14 in the transistor 10 thatoverlap with neither the conductive film 15 nor the conductive film 16(i.e., end portions of the semiconductor film 14 in a region differentfrom the region where the conductive film 15 and the conductive film 16are formed) are longer than end portions of the semiconductor film 24 inthe transistor 20 that overlap with neither the conductive film 25 northe conductive film 26 (i.e., end portions of the semiconductor film 24in a region different from the region where the conductive film 25 andthe conductive film 26 are formed).

When the lines of electric force of the transistor 10 in FIG. 2A arecompared to the lines of electric force of the transistor 20 in FIG. 2B,the density of lines of electric force in the end portions of thesemiconductor film 14 in the transistor 10 that overlap with neither theconductive film 15 nor the conductive film 16 can be lower than thedensity of lines of electric force in the end portions of thesemiconductor film 24 in the transistor 20 that overlap with neither theconductive film 25 nor the conductive film 26. In other words, anelectric field applied to the end portions of the semiconductor film 14in the region different from the region where the conductive film 15 andthe conductive film 16 are formed can be lower than an electric fieldapplied to the end portions of the semiconductor film 24 in the regiondifferent from the region where the conductive film 25 and theconductive film 26 are formed.

In the case where the semiconductor film 14 and the semiconductor film24 each include an oxide semiconductor, when the end portions of thesemiconductor film 14 and the semiconductor film 24 are exposed toplasma by etching for forming the end portions, chlorine radical,fluorine radical, or the like generated from an etching gas is easilybonded to a metal element contained in the oxide semiconductor. Thus, inthe end portions of the semiconductor film 14 and the semiconductor film24, oxygen bonded to the metal element is easily eliminated, so that anoxygen vacancy is easily formed.

However, in the transistor 10, the end portions of the semiconductorfilm 14 that overlap with neither the conductive film 15 nor theconductive film 16 can be made long as described above. Further, in thetransistor 10, in the region different from the region where theconductive film 15 and the conductive film 16 are formed, an electricfield applied to the end portions of the semiconductor film 14 can bedecreased. Accordingly, even when an oxygen vacancy is formed in the endportion of the semiconductor film 14, leakage current flowing betweenthe conductive film 15 and the conductive film 16 through the endportion can be reduced when the transistor 10 needs to be turned off.Consequently, the threshold voltage of the transistor 10 can becontrolled so that the transistor 10 is normally off.

In the transistor 10, by decreasing an electric field applied to the endportion of the semiconductor film 14, it is possible to prevent anelectron (carrier) from being trapped in the gate insulating film 13from the end portion. As a result, in the transistor 10, changes inthreshold voltage can be suppressed, so that the reliability of thesemiconductor device including the transistor 10 can be increased.

Further, in one embodiment of the present invention, a metal oxide maybe used for the oxide film 17.

The use of the oxide film 17 having such a structure can space thesemiconductor film 14 from a film containing silicon even when the filmcontaining silicon is provided over the oxide film 17. Thus, in the casewhere the semiconductor film 14 contains indium, silicon, which hashigher oxygen bond energy than indium, breaks the bond between indiumand oxygen in the end portions of the semiconductor film 14 that overlapwith neither the conductive film 15 nor the conductive film 16 and canprevent generation of oxygen vacancies. As a result, in one embodimentof the present invention, the reliability of the transistor can befurther increased.

In order to prevent a channel region of the semiconductor film 14 fromhaving n-type conductivity due to an oxygen vacancy, the concentrationof silicon in the semiconductor film 14 is preferably lower than orequal to 2×10¹⁸ atoms/cm³, more preferably lower than or equal to 2×10¹⁷atoms/cm³.

Note that the conductivity of the metal oxide is lower than that of ametal oxide used as an oxide semiconductor in the semiconductor film 14.In order to achieve such a structure, for example, in the case where anIn—Ga—Zn-based oxide is used as a metal oxide in the oxide film 17, themetal oxide in the oxide film 17 preferably has an atomic ratio where anatomic percent of In is lower than that of In in the atomic ratio of themetal oxide used for the semiconductor film 14. Specifically, the oxidefilm 17 can be formed by sputtering using an In—Ga—Zn-based oxide targethaving a metal atomic ratio of 1:6:4 or 1:3:2.

Note that FIG. 1A and FIG. 2A each illustrate an example in which theend portions of the semiconductor film 14 and the end portions of theconductive film 15 and the conductive film 16 in the region 18 have thespace Wd1 and the space Wd2. In one embodiment of the present invention,the above effect owing to one embodiment of the present invention can beobtained even when one of the space Wd1 and the space Wd2 does notexist. However, the structure examples of FIG. 1A and FIG. 2A where bothof the space Wd1 and the space Wd2 exist are preferable because theabove effect can be enhanced.

In the case where an oxide semiconductor is used for the semiconductorfilm 14, metal in the conductive film 15 and the conductive film 16extracts oxygen from the oxide semiconductor depending on a conductivematerial used for the conductive film 15 and the conductive film 16. Inthat case, a region in the semiconductor film 14 that is in contact withthe conductive film 15 and the conductive film 16 has n-typeconductivity due to generation of oxygen vacancies. FIG. 18 is amagnified view of a region 65 that is part of the transistor 10 in FIG.1A. In FIG. 18, a region 14 n in the semiconductor film 14 that is incontact with the conductive film 15 and the conductive film 16 hasn-type conductivity.

Since the region 14 n having n-type conductivity functions as a sourceregion or a drain region, contact resistance between the semiconductorfilm 14 and the conductive films 15 and 16 can be lowered. Thus, byforming the region 14 n having n-type conductivity, the mobility andon-state current of the transistor 10 can be increased, so that thesemiconductor device including the transistor 10 can operate at highspeed.

Note that metal in the conductive film 15 and the conductive film 16might extract oxygen when the conductive film 15 and the conductive film16 are formed by sputtering or the like or might extract oxygen by heattreatment performed after the conductive film 15 and the conductive film16 are formed.

Further, the region 14 n having n-type conductivity is easily formed byusing a conductive material that is easily bonded to oxygen for theconductive film 15 and the conductive film 16. The conductive materialcan be, for example, Al, Cr, Cu, Ta, Ti, Mo, or W.

<Amount of Change in Threshold Voltage>

Next, the amount of change in threshold voltage of the transistor 10 inFIG. 2A and the amount of change in threshold voltage of the transistor20 in FIG. 2B after high voltage is applied to the drain electrodes aredescribed.

First, a transistor A and a transistor B used in the test each have astructure similar to that of the transistor 10. The space Wd1 and thespace Wd2 are each 3 μm, the width Wsd of the conductive film 15 and theconductive film 16 is 20 μm, and a space (channel length) between theconductive film 15 and the conductive film 16 is 3 μm. Further, atransistor C and a transistor D used in the test each have a structuresimilar to that of the transistor 20. The space Wd3 and the space Wd4are each 3 ↑m, the width Wi of the semiconductor film 24 is 20 μm, and aspace (channel length) between the conductive film 25 and the conductivefilm 26 is 3 μm.

In each of the transistor A and the transistor B, a 200-nm-thicktungsten film is used as the conductive film 12. An insulating film inwhich a 400-nm-thick silicon nitride film and a 50-nm-thick siliconoxynitride film are sequentially stacked from the conductive film 12side is used as the gate insulating film. A conductive film in which a50-nm-thick tungsten film, a 400-nm-thick aluminum film, and a100-nm-thick titanium film are sequentially stacked from thesemiconductor film 14 side is used as the conductive film 15 and theconductive film 16.

Materials and thicknesses of the gate insulating films and theconductive films used in the transistor C and the transistor D are thesame as those in the transistor A and the transistor B. Specifically, ineach of the transistor C and the transistor D, a 200-nm-thick tungstenfilm is used as the conductive film 22. An insulating film in which a400-nm-thick silicon nitride film and a 50-nm-thick silicon oxynitridefilm are sequentially stacked from the conductive film 22 side is usedas the gate insulating film. A conductive film in which a 50-nm-thicktungsten film, a 400-nm-thick aluminum film, and a 100-nm-thick titaniumfilm are sequentially stacked from the semiconductor film 24 side isused as the conductive film 25 and the conductive film 26.

Note that in this specification, an oxynitride used for a siliconoxynitride film or the like is a substance that includes more oxygenthan nitrogen, and a nitride oxide is a substance that includes morenitrogen than oxygen.

As each of the semiconductor film 14 of the transistor A and thesemiconductor film 24 of the transistor C, a single-layer oxidesemiconductor film is used. The oxide semiconductor film is a35-nm-thick In—Ga—Zn-based oxide semiconductor film (IGZO (111)) formedusing an oxide target in which a composition ratio of indium (In) togallium (Ga) and zinc (Zn) is 1:1:1.

As each of the semiconductor film 14 of the transistor B and thesemiconductor film 24 of the transistor D, a two-layer oxidesemiconductor film is used. An oxide semiconductor film that is close tothe gate insulating film is a 35-nm-thick In—Ga—Zn-based oxidesemiconductor film (IGZO (111)) formed using an oxide target acomposition ratio of indium (In) to gallium (Ga) and zinc (Zn) is 1:1:1.An oxide semiconductor film that is far from the gate insulating film isa 20-nm-thick In—Ga—Zn-based oxide semiconductor film (IGZO (132))formed using an oxide target a composition ratio of indium (In) togallium (Ga) and zinc (Zn) is 1:3:2.

In the test, the voltage of the drain electrode (referred to as drainvoltage) at the time when the gate electrode and the source electrodehave the same potential (reference potential) is 30 V. Further, in thetest, stress is applied to the transistors A to D while the temperatureof a substrate provided with the transistors A to D is set to 125° C.and the transistors A to D are left in a darkroom for one hour withoutlight irradiation.

FIGS. 16A and 16B and FIGS. 17A and 17B show the relation between gatevoltage Vg (V) and drain current Id (A) of each of the transistors A toD that are measured before and after application of stress. Note that inFIGS. 16A and 16B and FIGS. 17A and 17B, the relation between the gatevoltage Vg and the drain current Id before application of stress isindicated by a broken line, and the relation between the gate voltage Vgand the drain current Id after application of stress is indicated by asolid line. FIG. 16A shows data of the transistor A, and FIG. 16B showsdata of the transistor B. FIG. 17A shows data of the transistor C, andFIG. 17B shows data of the transistor D.

Note that the drain current of each of the transistors A to D ismeasured by changing the gate voltage Vg from −15 V to +30 V. Themeasurement is performed with drain voltages Vd of 0.1 V and 10 V at 40°C.

FIG. 8 is a graph showing the amount of change in threshold voltage(ΔVth) and the amount of change in shift value (ΔShift) generated beforeand after application of stress that are calculated using the relationbetween the gate voltage Vg (V) and the drain current Id (A) shown inFIGS. 16A and 16B and FIGS. 17A and 17B. Note that the mobility of eachtransistor is calculated on the assumption that relative dielectricconstant is 4 and that the thickness of the gate insulating film is 280nm. Table 1 shows the amount of change in threshold voltage (ΔVth) andthe amount of change in shift value (ΔShift) generated before and afterapplication of stress.

TABLE 1 IGZO (111) IGZO (111)\IGZO (132) Transistor A Transistor CTransistor B Transistor D ΔVth ΔShift ΔVth ΔShift ΔVth ΔShift ΔVthΔShift −0.05 0.25 1.84 2.26 −0.77 −0.63 1.16 1.19

Note that the shift value is defined as the value of gate voltage at thetime when drain current rises. Specifically, in a graph showing therelation between gate voltage and drain current, the shift value can bedefined as voltage at an intersection of a tangent where slope change indrain current is the steepest and a graduation line corresponding to thelowest drain current. The shift value is a value at the time when drainvoltage is 10V.

As can be seen from FIG. 8, the amount of change in threshold voltage(ΔVth) and the amount of change in shift value (ΔShift) in thetransistor A and the transistor B each having the structure of thetransistor 10 are smaller than those in the transistor C and thetransistor D each having the structure of the transistor 20.Accordingly, the test results indicate that the threshold voltage of thetransistor 10 is less likely to be shifted in a positive direction andthe reliability of the transistor 10 is higher than that of thetransistor 20.

<Aspect 2 of Transistor>

Next, FIGS. 3A to 3D illustrate another aspect of a transistor includedin a semiconductor device according to one embodiment of the presentinvention. FIG. 3A is a top view of a transistor 30. FIG. 3B correspondsto a diagram illustrating a cross-sectional structure of the transistor30 in FIG. 3A taken along broken line B1-B2. FIG. 3C corresponds to adiagram illustrating a cross-sectional structure of the transistor 30 inFIG. 3A taken along broken line B3-B4. FIG. 3D corresponds to a diagramillustrating a cross-sectional structure of the transistor 30 in FIG. 3Ataken along broken line B5-B6. Note that insulating films such as a gateinsulating film are not illustrated in FIG. 3A in order to clarify thelayout of the transistor 30.

Like the transistor 10, the transistor 30 illustrated in FIGS. 3A to 3Dincludes, over a substrate 31 having an insulating surface, a conductivefilm 32 functioning as a gate electrode, a gate insulating film 33 overthe conductive film 32, a semiconductor film 34 overlapping with theconductive film 32 with the gate insulating film 33 positionedtherebetween, and a conductive film 35 and a conductive film 36 that arein contact with the semiconductor film 34 and function as a sourceelectrode and a drain electrode.

In FIGS. 3A to 3D, an oxide film 37 is provided over the semiconductorfilm 34, the conductive film 35, and the conductive film 36. In oneembodiment of the present invention, the oxide film 37 may be acomponent of the transistor 30.

The structure of the transistor 30 differs from the structure of thetransistor 10 illustrated in FIGS. 1A to 1C in that the conductive film35 and the conductive film 36 each have a comb-like shape. Specifically,a comb-like shape corresponds to a shape in which a plurality of convexportions are provided in end portions. The conductive film 35 and theconductive film 36 having a comb-like shape each include a plurality ofconvex portions 60 and a joint part 61 for coupling the plurality ofconvex portions 60.

In one embodiment of the present invention, in the transistor 30, an endportion of the semiconductor film 34 is spaced from an end portion ofthe conductive film 35 or the conductive film 36 in a region overlappingwith the semiconductor film 34 in a channel width direction indicated bythe arrow D2. From another perspective, it can be said that in thetransistor 30, the width Wi of the semiconductor film 34 in the channelwidth direction is larger than the width Wsd of the conductive film 35or the conductive film 36 in a region 38 where the conductive film 35 orthe conductive film 36 overlaps with the semiconductor film 34 in thechannel width direction.

In addition, in one embodiment of the present invention, the joint part61 of the conductive film 35 or the conductive film 36 is spaced fromthe end portion of the semiconductor film 34. In other words, in theconvex portion 60, the conductive film 35 or the conductive film 36partly overlaps with the semiconductor film 34. Thus, in the end portionof the conductive film 35 or the conductive film 36 in the regionoverlapping with the semiconductor film 34, the plurality of convexportions 60 are spaced from each other. Note that in order that thejoint part 61 of the conductive film 35 and the joint part 61 of theconductive film 36 are spaced from the end portions of the semiconductorfilm 34, in a channel length direction indicated by the arrow D1, aspace Lsd2 between the end portions of the joint parts of the conductivefilm 35 and the conductive film 36 needs to be larger than the width Liof the semiconductor film 34.

In one embodiment of the present invention, with a structure where thejoint part 61 of the conductive film 35 or the conductive film 36 isspaced from the end portion of the semiconductor film 34, the transistor30 can be normally off, and changes in threshold voltage can beprevented. The reason for this is described in detail below.

Lines of electric force (indicated by broken arrows) are shown betweenthe conductive film 35 and the conductive film 36 in the top view ofpart of the transistor 30 in FIG. 4. FIG. 4 illustrates lines ofelectric force at the time when the transistor 30 is an n-channeltransistor, the conductive film 35 is a drain electrode, and theconductive film 36 is a source electrode.

In the transistor 30, the end portion of the semiconductor film 34 isspaced from the end portion of the conductive film 35 or the conductivefilm 36 in the region overlapping with the semiconductor film 34 in thechannel width direction; thus, the end portions of the semiconductorfilm 34 that overlap with neither the conductive film 35 nor theconductive film 36 can be made long. Further, in the transistor 30, in aregion different from the region where the conductive film 35 and theconductive film 36 are formed, an electric field applied to the endportions of the semiconductor film 34 that can be a current path of theconductive film 35 and the conductive film 36 can be decreased.Accordingly, even when an oxygen vacancy is formed in the end portion ofthe semiconductor film 34, leakage current flowing between theconductive film 35 and the conductive film 36 through the end portioncan be reduced when the transistor 30 needs to be turned off.Consequently, the threshold voltage of the transistor 30 can becontrolled so that the transistor 30 is normally off.

In the transistor 30, by decreasing an electric field applied to the endportion of the semiconductor film 34, it is possible to prevent anelectron (carrier) from being trapped in the gate insulating film 33from the end portion. As a result, in the transistor 30, changes inthreshold voltage can be suppressed, so that the reliability of thesemiconductor device including the transistor 30 can be increased.

In the transistor 30 in FIG. 4, the lines of electric force extend fromthe conductive film 35 (drain electrode) to the conductive film 36(source electrode). In the transistor 30, the lines of electric forceexist in a region 39 a including a path through which the conductivefilm 35 is connected to the conductive film 36 in the channel lengthdirection indicated by the arrow D1 in the semiconductor film 34.Further, in the transistor 30, the lines of electric force exist notonly in the region 39 a but also in a region 39 b that is off the pathin the semiconductor film 34 to wrap around the conductive film 35 andthe conductive film 36.

Thus, in the case of the transistor 30, compared to the transistor wherethe joint part 61 of the conductive film 35 or the conductive film 36overlaps with the semiconductor film 34, the density of the lines ofelectric force that extend from the conductive film 35 to the conductivefilm 36 can be decreased. Accordingly, in the transistor 30, it ispossible to prevent concentration of an electric field not only in theend portion of the semiconductor film 34 but also in the inside of thesemiconductor film 34. As a result, in the transistor 30, changes inthreshold voltage can be suppressed, so that the reliability of thesemiconductor device can be increased.

In addition, in one embodiment of the present invention, the convexportion 60 of the conductive film 35 and the convex portion 60 of theconductive film 36 have the space Lsd1 in the channel length directionindicated by the arrow D1, and the convex portion 60 of the conductivefilm 35 and the convex portion 60 of the conductive film 36 do notinterlock with each other. With such a structure, the transistor 30 canhave a smaller area of a region where the conductive film 32 functioningas a gate electrode overlaps with the conductive film 35 or theconductive film 36, so that a capacitor formed in the region can be madesmall. Then, since the capacitor is small, the transistor 30 can have asmall subthreshold swing (S value).

Note that in the case of a transistor having a large subthreshold swing,if the threshold voltage becomes lower, the transistor is likely to benormally on where off-state current flowing at a gate voltage of 0 V ishigh. Thus, it is difficult for a circuit that includes transistorshaving the same conductivity type to operate correctly. Since thethreshold voltage of the transistor 30 can be lowered and thesubthreshold swing can be decreased, the transistor can be normally offmore reliably. As a result, by using the transistor 30, the circuit thatincludes transistors having the same conductivity type can operatecorrectly more reliably.

Compared to the transistor 10 illustrated in FIGS. 1A to 1C, an electrondepletion layer of the transistor 30 is likely to be spread on a backchannel side, that is, the center of the channel width of a region thatis near a surface of the semiconductor film 34 opposite to a surfacefacing the gate electrode when negative gate voltage is applied.Accordingly, the transistor 30 can have smaller effective channel widththan the transistor 10 illustrated in FIGS. 1A to 1C. Consequently, theregion through which off-state current flows is narrowed, so that theoff-state current can be reduced.

Further, in one embodiment of the present invention, a metal oxide maybe used for the oxide film 37.

The use of the oxide film 37 having such a structure can space thesemiconductor film 34 from a film containing silicon even when the filmcontaining silicon is provided over the oxide film 37. Thus, in the casewhere the semiconductor film 34 contains indium, silicon, which hashigher oxygen bond energy than indium, breaks the bond between indiumand oxygen in the end portions of the semiconductor film 34 that overlapwith neither the conductive film 35 nor the conductive film 36 and canprevent generation of oxygen vacancies. As a result, in one embodimentof the present invention, the reliability of the transistor can befurther increased.

Note that the conductivity of the metal oxide is lower than that of ametal oxide used as an oxide semiconductor in the semiconductor film 34.In order to achieve such a structure, for example, in the case where anIn—Ga—Zn-based oxide is used as a metal oxide in the oxide film 37, themetal oxide in the oxide film 37 preferably has an atomic ratio where anatomic percent of In is lower than that of In in the atomic ratio of themetal oxide used for the semiconductor film 34. Specifically, the oxidefilm 37 can be formed by sputtering using an In—Ga—Zn-based oxide targethaving a metal atomic ratio of 1:6:4 or 1:3:2.

Like the transistor 10, regions that are in contact with the conductivefilm 35 and the conductive film 36 in the semiconductor film 34 may haven-type conductivity. With such a structure, the mobility and on-statecurrent of the transistor 30 can be increased, so that the semiconductordevice including the transistor 30 can operate at high speed.

<Aspect 3 of Transistor>

Note that the transistor 30 illustrated in FIGS. 3A to 3D has astructure where the convex portion 60 of the conductive film 35completely overlaps with the convex portion 60 of the conductive film 36in the channel length direction. However, in one embodiment of thepresent invention, the convex portions 60 of the conductive film 35 andthe conductive film 36 may partly overlap with each other in the channellength direction.

FIG. 5A is a top view illustrating one aspect of the transistor 30 wherethe convex portions 60 partly overlap with each other in the channellength direction. The transistor 30 illustrated in FIG. 5A has astructure where the convex portion 60 of the conductive film 35 partlyoverlaps with the convex portion 60 of the conductive film 36 in thechannel length direction indicated by the arrow D1.

In the transistor 30 illustrated in FIGS. 3A to 3D, the conductive film35 and the conductive film 36 each include the plurality of convexportions 60. However, in the transistor 30, one of the conductive film35 and the conductive film 36 may include the plurality of convexportions 60.

FIG. 5B is a top view illustrating one aspect of the transistor 30 wherethe conductive film 35 include the plurality of convex portions 60 andthe conductive film 36 does not include the plurality of convex portions60. In FIG. 5B, unlike the end portion of the conductive film 35 in aregion overlapping with the semiconductor film 34, the end portion ofthe conductive film 36 in a region overlapping with the semiconductorfilm 34 is a series of end portions.

Even in the case of the transistor 30 illustrated in FIG. 5A and FIG.5B, an advantageous effect of one embodiment of the present inventioncan be obtained as in the case of the transistor 30 illustrated in FIGS.3A to 3D.

<Aspect 4 of Transistor>

Note that FIGS. 1A to 1C, FIGS. 2A and 2B, FIGS. 3A to 3D, FIG. 4, andFIGS. 5A and 5B each illustrate a bottom-gate transistor including asemiconductor film over a gate electrode. However, the transistorsillustrated in FIGS. 1A to 1C, FIGS. 2A and 2B, FIGS. 3A to 3D, FIG. 4,and FIGS. 5A and 5B may each be a top-gate transistor including asemiconductor film below a gate electrode.

FIGS. 6A to 6D illustrate one aspect of a top-gate transistor includedin a semiconductor device according to one embodiment of the presentinvention. FIG. 6A is a top view of a transistor 40. FIG. 6B correspondsto a diagram illustrating a cross-sectional structure of the transistor40 in FIG. 6A taken along broken line C1-C2. FIG. 6C corresponds to adiagram illustrating a cross-sectional structure of the transistor 40 inFIG. 6A taken along broken line C3-C4. FIG. 6D corresponds to a diagramillustrating a cross-sectional structure of the transistor 40 in FIG. 6Ataken along broken line C5-C6. Note that insulating films such as a gateinsulating film are not illustrated in FIG. 6A in order to clarify thelayout of the transistor 40.

The transistor 40 illustrated in FIGS. 6A to 6D includes, over asubstrate 41 having an insulating surface, a semiconductor film 44, aconductive film 45 and a conductive film 46 that function as a sourceelectrode and a drain electrode and are provided over the semiconductorfilm 44, a gate insulating film 43 over the semiconductor film 44, theconductive film 45, and the conductive film 46, and a conductive film 42that functions as a gate electrode and overlaps with the semiconductorfilm 44 with the gate insulating film 43 positioned therebetween.

In FIGS. 6A to 6D, an oxide film 47 is provided over the gate insulatingfilm 43 and the conductive film 42. In one embodiment of the presentinvention, the oxide film 47 may be a component of the transistor 40.

The structure of the transistor 40 is the same as the structure of thetransistor 30 illustrated in FIGS. 3A to 3D in that the conductive film45 and the conductive film 46 each have a comb-like shape. Theconductive film 45 and the conductive film 46 having a comb-like shapeeach include a plurality of convex portions 50 and a joint part 51 forcoupling the plurality of convex portions 50.

The structure of the transistor 40 is the same as the structure of thetransistor 30 illustrated in FIGS. 3A to 3D in that an end portion ofthe semiconductor film 44 is spaced from an end portion of theconductive film 45 or the conductive film 46 in a region overlappingwith the semiconductor film 44 in a channel width direction indicated bythe arrow D2. From another perspective, it can be said that in thetransistor 40, the width Wi of the semiconductor film 44 in the channelwidth direction is larger than the width Wsd of the conductive film 45or the conductive film 46 in a region 48 where the conductive film 45 orthe conductive film 46 overlaps with the semiconductor film 44 in thechannel width direction.

The structure of the transistor 40 is also the same as the structure ofthe transistor 30 illustrated in FIGS. 3A to 3D in that the joint part51 of the conductive film 45 or the conductive film 46 is spaced fromthe end portion of the semiconductor film 44. Thus, in the end portionof the conductive film 45 or the conductive film 46 in the regionoverlapping with the semiconductor film 44, the plurality of convexportions 50 are spaced from each other. Note that in order that thejoint part 51 of the conductive film 45 and the joint part 51 of theconductive film 46 are spaced from the end portions of the semiconductorfilm 44, in a channel length direction indicated by the arrow D1, aspace Lsd2 between the end portions of the joint parts of the conductivefilm 45 and the conductive film 46 needs to be larger than the width Liof the semiconductor film 44.

Note that like the transistor 10, regions that are in contact with theconductive film 45 and the conductive film 46 in the semiconductor film44 may have n-type conductivity. With such a structure, the mobility andon-state current of the transistor 40 can be increased, so that thesemiconductor device including the transistor 40 can operate at highspeed.

<Layout of Transistors>

FIG. 7A illustrates an example of a top view of the two transistors 30illustrated in FIGS. 3A to 3D connected to each other in parallel.

Note that in this specification, a state in which transistors areconnected to each other in series means, for example, a state in whichonly one of a source electrode and a drain electrode of a firsttransistor is connected to only one of a source electrode and a drainelectrode of a second transistor. In addition, a state in whichtransistors are connected to each other in parallel means a state inwhich one of a source electrode and a drain electrode of a firsttransistor is connected to one of a source electrode and a drainelectrode of a second transistor and the other of the source electrodeand the drain electrode of the first transistor is connected to theother of the source electrode and the drain electrode of the secondtransistor.

FIG. 7A illustrates the two transistors 30 illustrated in FIGS. 3A to 3Das a transistor 30 a and a transistor 30 b. The conductive film 35 ofthe transistor 30 a and the conductive film 35 of the transistor 30 bshare the joint part 61. The semiconductor film 34 of the transistor 30a and the semiconductor film 34 of the transistor 30 b are positioned insuch a way that a channel length direction indicated by the arrow D1 anda channel width direction indicated by the arrow D2 of the transistor 30a are substantially aligned with those of the transistor 30 b.

FIG. 7B illustrates the two transistors 40 illustrated in FIGS. 6A to 6Das a transistor 40 a and a transistor 40 b. The conductive film 45 ofthe transistor 40 a and the conductive film 45 of the transistor 40 bshare the joint part 51. The semiconductor film 44 of the transistor 40a and the semiconductor film 44 of the transistor 40 b are positioned insuch a way that a channel length direction indicated by the arrow D1 anda channel width direction indicated by the arrow D2 of the transistor 40a are substantially aligned with those of the transistor 40 b.

Note that although FIGS. 7A and 7B each illustrate the example in whichtwo transistors are connected to each other in parallel, three or moretransistors can be similarly connected to each other in parallel.

As illustrated in FIGS. 7A and 7B, by providing the plurality oftransistors 30 or 40, the proportion of a regular pattern in layout of amask used for the plurality of transistors 30 or 40 can be increased. Inthe case where the proportion of the regular pattern of the mask is low,shape defects are likely to occur in a photolithography process usingthe mask because of interference of light emitted from an exposureapparatus, for example, the width of a conductive film, an insulatingfilm, a semiconductor film, or the like processed by photolithography ispartly narrow. However, in FIGS. 7A and 7B, the proportion of theregular pattern in layout of the mask used for the plurality oftransistors 30 or 40 can be increased, so that it is possible to preventgeneration of shape defects in a conductive film, an insulating film, ora semiconductor film after a photolithography process.

<Semiconductor Film>

In a semiconductor device according to one embodiment of the presentinvention, a semiconductor film containing amorphous, microcrystalline,polycrystalline, or single crystal silicon, germanium, or the like maybe used as a semiconductor film of a transistor. Alternatively, asemiconductor film containing a semiconductor such as an oxidesemiconductor whose bandgap is wider than that of silicon and whoseintrinsic carrier density is lower than that of silicon may be used.

Any of the following can be used as silicon: amorphous silicon formed bysputtering or vapor deposition such as plasma-enhanced CVD;polycrystalline silicon obtained in such a manner that amorphous siliconis crystallized by laser annealing or the like; single crystal siliconobtained in such a manner that a surface portion of a single crystalsilicon wafer is separated by implantation of hydrogen ions or the likeinto the silicon wafer; and the like.

A highly-purified oxide semiconductor (purified oxide semiconductor)obtained by reduction of impurities such as moisture or hydrogen thatserve as electron donors (donors) and reduction of oxygen vacancies isan intrinsic (i-type) semiconductor or a substantially intrinsicsemiconductor. Thus, a transistor including a channel formation regionin a highly-purified oxide semiconductor film has extremely lowoff-state current and high reliability.

Specifically, various experiments can prove low off-state current of atransistor including a channel formation region in a highly-purifiedoxide semiconductor film. For example, even when an element has achannel width of 1×10⁶ μm and a channel length of 10 μm, off-statecurrent can be lower than or equal to the measurement limit of asemiconductor parameter analyzer, i.e., lower than or equal to 1×10⁻¹³A, at a voltage (drain voltage) between a source electrode and a drainelectrode of 1 to 10 V. In that case, it can be seen that off-statecurrent standardized on the channel width of the transistor is lowerthan or equal to 100 zA/μm. In addition, a capacitor and a transistorwere connected to each other and off-state current was measured using acircuit in which electric charge flowing to or from the capacitor iscontrolled by the transistor. In the measurement, a highly-purifiedoxide semiconductor film was used in the channel formation region of thetransistor, and the off-state current of the transistor was measuredfrom a change in the amount of electric charge of the capacitor per unithour. As a result, it can be seen that, in the case where the voltagebetween the source electrode and the drain electrode of the transistoris 3 V, a lower off-state current of several tens of yoctoamperes permicrometer is obtained. Accordingly, the transistor including thehighly-purified oxide semiconductor film in the channel formation regionhas much lower off-state current than a crystalline silicon transistor.

Note that unless otherwise specified, in this specification, off-statecurrent of an n-channel transistor is current that flows between asource and a drain when the potential of the drain is higher than thatof the source or that of a gate while the potential of the gate is 0 Vor lower in the case of the potential of the source used as a reference.Alternatively, in this specification, off-state current of a p-channeltransistor is current that flows between a source and a drain when thepotential of the drain is lower than that of the source or that of agate while the potential of the gate is 0 V or higher in the case of thepotential of the source used as a reference.

In the case where an oxide semiconductor film is used as thesemiconductor film, an oxide semiconductor preferably contains at leastindium (In) or zinc (Zn). As a stabilizer for reducing variations inelectrical characteristics of a transistor including the oxidesemiconductor, the oxide semiconductor preferably contains gallium (Ga)in addition to In and Zn. Tin (Sn) is preferably contained as astabilizer. Hafnium (Hf) is preferably contained as a stabilizer.Aluminum (Al) is preferably contained as a stabilizer. Zirconium (Zr) ispreferably contained as a stabilizer.

Among the oxide semiconductors, unlike silicon carbide, gallium nitride,or gallium oxide, an In—Ga—Zn-based oxide, an In—Sn—Zn-based oxide, orthe like has an advantage of high mass productivity because a transistorwith favorable electrical characteristics can be formed by sputtering ora wet process. Further, unlike silicon carbide, gallium nitride, orgallium oxide, with the use of the In—Ga—Zn-based oxide, a transistorwith favorable electrical characteristics can be formed over a glasssubstrate. Furthermore, a larger substrate can be used.

As another stabilizer, one or more kinds of lanthanoid such as lanthanum(La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) may becontained.

For example, indium oxide, gallium oxide, tin oxide, zinc oxide, anIn—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, aZn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, anIn—Ga-based oxide, an In—Ga—Zn-based oxide (also referred to as IGZO),an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-basedoxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, anIn—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Pr—Zn-based oxide,an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-basedoxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, anIn—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide,an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, an In—Lu—Zn-basedoxide, an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide can be used as anoxide semiconductor.

Note that, for example, an In—Ga—Zn-based oxide means an oxidecontaining In, Ga, and Zn, and there is no limitation on the ratio ofIn, Ga, and Zn. In addition, the In—Ga—Zn-based oxide may contain ametal element other than In, Ga, and Zn. The In—Ga—Zn-based oxide hassufficiently high resistance when no electric field is applied thereto,so that off-state current can be sufficiently reduced. Further, theIn—Ga—Zn-based oxide has high mobility.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), or anoxide whose composition is in the neighborhood of the above compositioncan be used. Alternatively, an In—Sn—Zn-based oxide with an atomic ratioof In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3 (=1/3:1/6:1/2), orIn:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or an oxide whose composition is in theneighborhood of the above composition is preferably used.

For example, with an In—Sn—Zn-based oxide, high mobility can becomparatively easily obtained. However, even with an In—Ga—Zn-basedoxide, mobility can be increased by lowering defect density in a bulk.

An oxide semiconductor film is roughly classified into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm. The non-single-crystal oxide semiconductor film means any of anamorphous oxide semiconductor film, a microcrystalline oxidesemiconductor film, a polycrystalline oxide semiconductor film, a c-axisaligned crystalline oxide semiconductor (CAAC-OS) film, and the like.

The amorphous oxide semiconductor film has disordered atomic arrangementand no crystalline component. A typical example of the amorphous oxidesemiconductor film is an oxide semiconductor film in which no crystalpart exists even in a microscopic region, and the whole of the film isamorphous.

The microcrystalline oxide semiconductor film includes a microcrystal(also referred to as nanocrystal) of greater than or equal to 1 nm andless than 10 nm, for example. Thus, the microcrystalline oxidesemiconductor film has higher degree of atomic order than the amorphousoxide semiconductor film. Hence, the density of defect states of themicrocrystalline oxide semiconductor film is lower than that of theamorphous oxide semiconductor film.

The CAAC-OS film is one of oxide semiconductor films including aplurality of crystal parts, and most of the crystal parts each fit intoa cube whose one side is less than 100 nm. Thus, there is a case where acrystal part included in the CAAC-OS film fits into a cube whose oneside is less than 10 nm, less than 5 nm, or less than 3 nm. The densityof defect states of the CAAC-OS film is lower than that of themicrocrystalline oxide semiconductor film. The CAAC-OS film is describedin detail below.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film.

In this specification, the term “parallel” indicates that an angleformed between two straight lines is −10 to 10°, and accordinglyincludes the case where the angle is −5 to 5°. In addition, the term“perpendicular” indicates that an angle formed between two straightlines is 80 to 100°, and accordingly includes the case where the angleis 85 to 95°.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (planar TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

From the results of the cross-sectional TEM image and the planar TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (φ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (φaxis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when φ scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer which is arranged in a layeredmanner and observed in the cross-sectional TEM image corresponds to aplane parallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of a formation surface or a normalvector of a top surface. Thus, for example, in the case where the shapeof the CAAC-OS film is changed by etching or the like, the c-axis mightnot be necessarily parallel to a normal vector of a formation surface ora normal vector of a top surface of the CAAC-OS film.

Further, the crystallinity in the CAAC-OS film is not necessarilyuniform. For example, in the case where crystal growth leading to theCAAC-OS film occurs from the vicinity of the top surface of the film,the crystallinity in the vicinity of the top surface is higher than thatin the vicinity of the formation surface in some cases. Further, when animpurity is added to the CAAC-OS film, the crystallinity in a region towhich the impurity is added is changed, and the crystallinity in theCAAC-OS film varies depending on regions.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ do not appear at around36°.

In a transistor including the CAAC-OS film, changes in electricalcharacteristics of the transistor due to irradiation with visible lightor ultraviolet light are small. Thus, the transistor has highreliability.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, amicrocrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

For example, a CAAC-OS film is deposited by sputtering with apolycrystalline metal oxide target. When ions collide with the target, acrystal region included in the target might be separated from the targetalong the a-b plane, and a sputtered particle having a plane parallel tothe a-b plane (a flat-plate-like sputtered particle or a pellet-likesputtered particle) might be separated from the target. In that case,the flat-plate-like sputtered particle reaches a substrate whilemaintaining its crystal state, so that the CAAC-OS film can bedeposited.

For the deposition of the CAAC-OS film, the following conditions arepreferably employed.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, or nitrogen) which exist in a treatmentchamber may be reduced. Further, the concentration of impurities in adeposition gas may be reduced. Specifically, a deposition gas whose dewpoint is −80° C. or lower, preferably −100° C. or lower is used.

By increasing the substrate heating temperature during the deposition,migration of a sputtered particle occurs after the sputtered particlereaches the substrate. Specifically, the substrate heating temperatureduring the deposition is 100 to 740° C., preferably 200 to 500° C. Byincreasing the substrate heating temperature during the deposition, whenthe flat-plate-like sputtered particle reaches the substrate, migrationoccurs on the substrate, so that a flat plane of the sputtered particleis attached to the substrate.

Further, it is preferable to reduce plasma damage during the depositionby increasing the proportion of oxygen in the deposition gas andoptimizing power. The proportion of oxygen in the deposition gas is 30vol % or higher, preferably 100 vol %.

As an example of the target, an In—Ga—Zn-based oxide target is describedbelow.

A polycrystalline In—Ga—Zn-based oxide target is made by mixing InO_(x)powder, GaO_(y) powder, and ZnO_(z) powder in a predetermined moleratio, applying pressure, and performing heat treatment at 1000 to 1500°C. Note that X, Y, and Z are each a given positive number. Here, thepredetermined mole ratio of the InO_(x) powder, the GaO_(y) powder, andthe ZnO_(z) powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3,or 3:1:2. The kinds of powder and the mole ratio for mixing powder maybe changed as appropriate depending on a target to be formed.

The semiconductor film is not necessarily a single oxide semiconductorfilm, but may be a stack of a plurality of oxide semiconductor films.FIG. 9 illustrates a structure example of a transistor 100 including asemiconductor film which is a stack of three oxide semiconductor films.

The transistor 100 illustrated in FIG. 9 includes, over a substrate 111having an insulating surface, a conductive film 112 functioning as agate electrode, a gate insulating film 113 over the conductive film 112,a semiconductor film 114 overlapping with the conductive film 112 withthe gate insulating film 113 positioned therebetween, and a conductivefilm 115 and a conductive film 116 that are in contact with thesemiconductor film 114 and function as a source electrode and a drainelectrode.

In FIG. 9, an oxide film 117 is provided over the semiconductor film114, the conductive film 115, and the conductive film 116. In oneembodiment of the present invention, the oxide film 117 may be acomponent of the transistor 100.

In the transistor 100, oxide semiconductor films 114 a to 114 c aresequentially stacked from the conductive film 112 side functioning as agate electrode.

Each of the oxide semiconductor films 114 a and 114 c is an oxide filmthat contains at least one of metal elements contained in the oxidesemiconductor film 114 b and in which energy at the bottom of theconduction band is closer to the vacuum level than that in the oxidesemiconductor film 114 b by higher than or equal to 0.05 eV, 0.07 eV,0.1 eV, or 0.15 eV and lower than or equal to 2 eV, 1 eV, 0.5 eV, or 0.4eV. The oxide semiconductor film 114 b preferably contains at leastindium because carrier mobility is increased.

In the case where the transistor 100 has such a structure, when anelectric field is applied to the semiconductor film 114 by applicationof voltage to the conductive film 112 functioning as a gate electrode, achannel region is formed in the oxide semiconductor film 114 b whoseenergy at the bottom of the conduction band is low in the semiconductorfilm 114. In other words, the oxide semiconductor film 114 c is providedbetween the oxide semiconductor film 114 b and the gate insulating film113, so that a channel region can be formed in the oxide semiconductorfilm 114 b spaced from the gate insulating film 113.

Since the oxide semiconductor film 114 c contains at least one of themetal elements contained in the oxide semiconductor film 114 b,interface scattering hardly occurs at an interface between the oxidesemiconductor films 114 b and 114 c. Thus, carriers are not easilyinhibited from moving at the interface, which results in an increase infield-effect mobility of the transistor 100.

When an interface state is formed at an interface between the oxidesemiconductor films 114 b and 114 a, a channel region is also formed ina region close to the interface; thus, the threshold voltage of thetransistor 100 varies. However, since the oxide semiconductor film 114 acontains at least one of the metal elements contained in the oxidesemiconductor film 114 b, an interface state is hardly formed at theinterface between the oxide semiconductor films 114 b and 114 a. As aresult, such a structure can reduce variations in electricalcharacteristics (e.g., threshold voltage) of the transistor 100.

The plurality of oxide semiconductor films are preferably stacked sothat impurities between the oxide semiconductor films do not form aninterface state that inhibits carriers from moving at an interface ofeach film. If impurities exist between the plurality of stacked oxidesemiconductor films, the continuity of energy at the bottom of theconduction band between the oxide semiconductor films is lost, andcarriers are trapped or lost due to recombination around the interface.A continuous bond (especially, a bond having a U-shaped and well-shapedstructure where energy at the bottom of the conduction band iscontinuously changed between the films) is more likely to be formed inthe plurality of oxide semiconductor films containing at least one metalelement (main component) in which impurities between the films arereduced than in the plurality of oxide semiconductor films that containat least one metal element (main component) and are simply stacked.

In order to form such a continuous bond, it is necessary to form filmscontinuously without being exposed to the atmosphere with the use of amulti-chamber deposition apparatus (sputtering apparatus) including aload lock chamber. Each chamber of the sputtering apparatus ispreferably evacuated to a high vacuum (to about 1×10⁻⁴ to 5×10⁻⁷ Pa) byan adsorption vacuum pump such as a cryopump so that water and the like,which are impurities for an oxide semiconductor, are removed as much aspossible. Alternatively, a turbo molecular pump and a cold trap arepreferably used in combination to prevent backflow of gas into thechamber through an evacuation system.

To obtain a highly-purified intrinsic oxide semiconductor, not only highvacuum evacuation of the chambers but also high purification of asputtering gas is important. An oxygen gas or an argon gas used as thegas is highly purified to have a dew point of −40° C. or lower,preferably −80° C. or lower, more preferably −100° C. or lower, so thatentry of moisture or the like into the oxide semiconductor film can beprevented as much as possible.

The oxide semiconductor film 114 a or 114 c may be, for example, anoxide film containing aluminum, silicon, titanium, gallium, germanium,yttrium, zirconium, tin, lanthanum, cerium, or hafnium at a higheratomic ratio than the oxide semiconductor film 114 b. Specifically, anoxide film containing the above element at an atomic ratio 1.5 or moretimes, preferably 2 or more times, more preferably 3 or more times thatin the oxide semiconductor film 114 b is preferably used as the oxidesemiconductor film 114 a or 114 c. The above element is strongly bondedto oxygen, and thus has a function of inhibiting generation of oxygenvacancies in the oxide film. Accordingly, with such a structure, theoxide semiconductor film 114 a or 114 c can be an oxide film in whichoxygen vacancies are less likely to be generated than in the oxidesemiconductor film 114 b.

Specifically, in the case where the oxide semiconductor film 114 b andthe oxide semiconductor film 114 a or 114 c are formed using anIn-M-Zn-based oxide, if the atomic ratio of the oxide semiconductor film114 a or 114 c is In:M:Zn=x₁:y₁:z₁ and the atomic ratio of the oxidesemiconductor film 114 b is In:M:Zn=x₂:y₂:z₂, the atomic ratios may beset so that y₁/x₁ is larger than y₂/x₂. Note that the element M is ametal element whose bonding strength to oxygen is larger than that ofIn, and can be Al, Ti, Ga, Y, Zr, Sn, La, Ce, Nd, or Hf, for example.Preferably, the atomic ratios may be set so that y₁/x₁ is 1.5 or moretimes y₂/x₂. More preferably, the atomic ratios may be set so that y₁/x₁is 2 or more times y₂/x₂. Still more preferably, the atomic ratios maybe set so that y₁/x₁ is 3 or more times y₂/x₂. In the oxidesemiconductor film 114 b, y₁ is preferably larger than or equal to x₁because the transistor 100 can have stable electrical characteristics.Note that y₁ is preferably less than 3 times x₁ because the field-effectmobility of the transistor 100 is lowered if y₁ is 3 or more times x₁.

FIG. 15A schematically shows part of a band structure at the time when asilicon oxide film is provided to be in contact with the stacked oxidesemiconductor films 114 a to 114 c. In FIG. 15A, the vertical axisrepresents electron energy (eV), and the horizontal axis representsdistance. In addition, EcI1 and EcI2 represent energies at the bottom ofthe conduction band of the silicon oxide film; EcS1 represents energy atthe bottom of the conduction band of the oxide semiconductor film 114 a;EcS2 represents energy at the bottom of the conduction band of the oxidesemiconductor film 114 b; and EcS3 represents energy at the bottom ofthe conduction band of the oxide semiconductor film 114 c.

As shown in FIG. 15A, the energies at the bottom of the conduction bandare continuously changed in the oxide semiconductor films 114 a to 114c. This is because the compositions of the oxide semiconductor films 114a to 114 c are close to each other and oxygen is easily diffused intothe oxide semiconductor films 114 a to 114 c.

Note that although FIG. 15A shows the case where the oxide semiconductorfilms 114 a and 114 c have similar energy gaps, the oxide semiconductorfilms 114 a and 114 c may have different energy gaps. For example, inthe case where EcS1 is higher than EcS3, part of the band structure canbe shown as in FIG. 15B. Although not shown in FIGS. 15A and 15B, EcS3may be higher than EcS1.

Note that as shown in FIGS. 15A and 15B, trap states resulting fromimpurities or defects can be formed in the vicinity of the interfacesbetween the oxide semiconductor films 114 a and 114 c and insulatingfilms such as a silicon oxide film. The oxide semiconductor films 114 aand 114 c make the oxide semiconductor film 114 b be separated from thetrap states. However, when the energy gap between EcS1 or EcS3 and EcS2is small, an electron in the oxide semiconductor film 114 b might reachthe trap level over the energy gap. Since the electron is trapped in thetrap level, negative fixed electric charge is caused at the interfacewith the insulating film; thus, the threshold voltage of the transistoris shifted in a positive direction.

Thus, the energy gap between EcS 1 and EcS2 and the energy gap betweenEcS3 and EcS2 are each preferably higher than or equal to 0.1 eV, morepreferably higher than or equal to 0.15 eV because the amount of changein threshold voltage of the transistor can be reduced and the transistorcan have stable electrical characteristics.

Note that the thickness of each of the oxide semiconductor films 114 aand 114 c is greater than or equal to 3 nm and less than or equal to 100nm, preferably greater than or equal to 3 nm and less than or equal to50 nm. The thickness of the oxide semiconductor film 114 b is greaterthan or equal to 3 nm and less than or equal to 200 nm, preferablygreater than or equal to 3 nm and less than or equal to 100 nm, morepreferably greater than or equal to 3 nm and less than or equal to 50nm.

The three oxide semiconductor films (oxide semiconductor films 114 a to114 c) can be either amorphous or crystalline. Note that the oxidesemiconductor film 114 b in which a channel region is formed ispreferably crystalline because the transistor 100 can have stableelectrical characteristics.

Note that a channel formation region means a region of a semiconductorfilm of a transistor that overlaps with a gate electrode and is betweena source electrode and a drain electrode. Further, a channel regionmeans a region through which current mainly flows in the channelformation region.

For example, in the case where an In—Ga—Zn-based oxide film formed bysputtering is used as each of the oxide semiconductor films 114 a and114 c, a target of an In—Ga—Zn-based oxide (In:Ga:Zn=1:3:2 [atomicratio]) can be used for deposition of the oxide semiconductor films 114a and 114 c. The deposition conditions can be, for example, as follows:an argon gas (flow rate: 30 sccm) and an oxygen gas (flow rate: 15 sccm)are used as a deposition gas; pressure is 0.4 Pa; substrate temperatureis 200° C.; and DC power is 0.5 kW.

In the case where the oxide semiconductor film 114 b is a CAAC-OS film,a target containing a polycrystalline In—Ga—Zn-based oxide(In:Ga:Zn=1:1:1 [atomic ratio]) is preferably used for the deposition.The deposition conditions can be, for example, as follows: an argon gas(flow rate: 30 sccm) and an oxygen gas (flow rate: 15 sccm) are used asa deposition gas; pressure is 0.4 Pa; substrate temperature is 300° C.;and DC power is 0.5 kW.

Note that the transistor 100 illustrated in FIG. 9 may have a structurewhere the end portion of the semiconductor film 114 is steep or astructure where the end portion of the semiconductor film 114 isrounded.

Note that although FIG. 9 illustrates the semiconductor film 114 formedusing a stack of three oxide semiconductor films, the number of stackedoxide semiconductor films may be 2 or more than 3.

Note that in the case where the semiconductor film 114 is formed using astack of a plurality of oxide semiconductor films, the conductivity of ametal oxide used for the oxide film 117 is lower than the totalconductivity of the semiconductor film 114. For example, in the casewhere an In—Ga—Zn-based oxide is used for the oxide film 117, the metaloxide used for the oxide film 117 preferably has an atomic ratio wherean atomic percent of In is lower than that of In in the atomic ratio ofthe metal oxide used for the semiconductor film 114.

Like the transistor 10, regions that are in contact with the conductivefilm 115 and the conductive film 116 in the semiconductor film 114 mayhave n-type conductivity. With such a structure, the mobility andon-state current of the transistor 100 can be increased, so that thesemiconductor device including the transistor 100 can operate at highspeed. In the case of the transistor 100, the regions having n-typeconductivity preferably extend to the oxide semiconductor film 114 bserving as a channel region in order that the mobility and on-statecurrent of the transistor 100 can be further increased and thesemiconductor device can operate at higher speed.

<Method for Forming Semiconductor Device>

An example of a method for forming a semiconductor device according toone embodiment of the present is described below.

As illustrated in FIG. 10A, a conductive film 201 is formed over asubstrate 200.

A substrate having heat resistance high enough to withstand a latermanufacturing step is preferably used as the substrate 200, and forexample, a glass substrate, a ceramic substrate, a quartz substrate, asapphire substrate, or the like is used.

A single layer or two or more layers of a film including a conductivematerial containing one or more kinds of aluminum, titanium, chromium,cobalt, nickel, copper, yttrium, zirconium, molybdenum, ruthenium,silver, tantalum, and tungsten are preferably formed as the conductivefilm 201. For example, a film in which a copper film is stacked over atungsten nitride film or a single layer film of tungsten can be formedas the conductive film 201.

Next, a conductive film 202 functioning as a gate electrode of thetransistor is formed by a photolithography process and an etchingprocess. Specifically, a mask formed using a resist (hereinafterreferred to as a resist mask) is formed over the conductive film 201 byusing a first photomask and then the conductive film 201 is etched, sothat a conductive film 202 is formed. Then, the resist mask is removed(see FIG. 10B).

Then, a gate insulating film 203 is formed to cover the conductive film202, and a semiconductor film 204 is formed over the gate insulatingfilm 203 (see FIG. 10C).

The gate insulating film 203 may be a single layer or a stacked layerusing an insulating film containing one or more of aluminum oxide,magnesium oxide, silicon oxide, silicon oxynitride, silicon nitrideoxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, andtantalum oxide.

For example, in the case of the gate insulating film 203 having atwo-layer structure, a multilayer film including a silicon nitride filmas a first layer and a silicon oxide film as a second layer may be used.The silicon oxide film as the second layer can be a silicon oxynitridefilm. The silicon nitride film as the first layer can be a siliconnitride oxide film.

It is preferable to use a silicon oxide film whose defect density is lowas the silicon oxide film. Specifically, a silicon oxide film whose spindensity attributed to a signal with a g factor of 2.001 in electron spinresonance (ESR) is lower than or equal to 3×10¹⁷ spins/cm³, preferablylower than or equal to 5×10¹⁶ spins/cm³ is used. As the silicon oxidefilm, a silicon oxide film having excess oxygen is preferably used. Asthe silicon nitride film, a silicon nitride film from which hydrogen andammonia are less released is used. The amount of released hydrogen andammonia is preferably measured by thermal desorption spectroscopy (TDS)analysis.

Next, the semiconductor film 204 is processed into a desired shape by aphotolithography process and an etching process, so that a semiconductorfilm 205 is formed (see FIG. 10D). Specifically, a resist mask is formedover the semiconductor film 204 by using a second photomask and thesemiconductor film 204 is etched, so that the semiconductor film 205 isformed. Then, the resist mask is removed.

As the oxide semiconductor film 205, the oxide semiconductor describedabove can be used.

Further, when the oxide semiconductor film used as the semiconductorfilm 205 contains a large amount of hydrogen, hydrogen and an oxidesemiconductor are bonded to each other, so that part of hydrogen servesas a donor and causes an electron (carrier). As a result, the thresholdvoltage of the transistor is shifted in a negative direction. Thus, itis preferable that after formation of the oxide semiconductor film,dehydration treatment (dehydrogenation treatment) be performed to removehydrogen or moisture from the oxide semiconductor film so that the oxidesemiconductor film contains impurities as little as possible.

Note that oxygen in the oxide semiconductor film is reduced by thedehydration treatment (dehydrogenation treatment) in some cases. Thus,it is preferable that oxygen be added to the oxide semiconductor film tofill oxygen vacancies increased by the dehydration treatment(dehydrogenation treatment).

In this manner, hydrogen or moisture is removed from the oxidesemiconductor film by dehydration treatment (dehydrogenation treatment)and oxygen vacancies are filled by oxygen adding treatment, so that theoxide semiconductor film can be an intrinsic (i-type) or substantiallyintrinsic oxide semiconductor film.

Then, a conductive film 206 is formed over the semiconductor film 205and the gate insulating film 203. The conductive film 206 can be formedusing the same conductive material as the conductive film 201 (see FIG.11A).

Next, a resist mask is formed over the conductive film 206 and the gateinsulating film 203 by using a third photomask. The conductive film 206is etched using this resist mask, so that a conductive film 207 and aconductive film 208 that are in contact with the semiconductor film 205are formed (see FIG. 11B).

Then, insulating films are formed to cover the entire substrate 200. InFIG. 11C, an oxide film 209, an insulating film 210, and an insulatingfilm 211 are formed.

A metal oxide is preferably used for the oxide film 209. The use of theoxide film 209 having such a structure can space the semiconductor film205 from the insulating film 210 containing silicon. Thus, in the casewhere a metal oxide containing indium is used for the semiconductor film205, silicon, which has higher oxygen bond energy than indium, breaksthe bond between indium and oxygen in end portions of the semiconductorfilm 205 and can prevent generation of oxygen vacancies. As a result, inone embodiment of the present invention, the reliability of thetransistor can be further increased.

Specifically, the oxide film 209 can be formed by sputtering using anIn—Ga—Zn-based oxide target having a metal atomic ratio of 1:6:4 or1:3:2.

It is preferable to form the insulating film 211 without exposure to theatmosphere, directly after the insulating film 210 is formed. Theinsulating film 211 is formed directly after the insulating film 210 isformed, by adjusting at least one of the flow rate of the source gas,the pressure, the high-frequency power, and the substrate temperaturewithout exposure to the atmosphere, so that the concentration ofimpurities at the interface between the insulating film 210 and theinsulating film 211 can be reduced and oxygen contained in theinsulating film 211 can move to the oxide semiconductor film 205.Accordingly, the amount of oxygen vacancies in the oxide semiconductorfilm 205 can be reduced.

As the insulating film 210, a silicon oxide film or a silicon oxynitridefilm is formed under the following conditions: the substrate placed in atreatment chamber of a plasma-enhanced CVD apparatus that isvacuum-evacuated is held at 180 to 400° C., preferably 200 to 370° C.,the pressure in the treatment chamber is 30 to 250 Pa, preferably 40 to200 Pa with introduction of a source gas into the treatment chamber, andhigh-frequency power is supplied to an electrode provided in thetreatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gases of the insulating film 210. Typical examples ofthe deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone,dinitrogen monoxide, nitrogen dioxide, or the like can be used.

Under the above conditions, an oxide insulating film that passes oxygencan be formed as the insulating film 210. With the insulating film 210,damage to the oxide film 209 can be reduced during a later formationprocess of the insulating film 211.

Note that when the ratio of the amount of the oxidizing gas to theamount of the deposition gas containing silicon is higher than or equalto 100, the hydrogen content in the insulating film 210 can be reduced,and dangling bonds in the insulating film 210 can be reduced. Oxygenthat moves from the insulating film 211 might be captured by thedangling bond in the insulating film 210. Thus, oxygen contained in theinsulating film 211 containing oxygen at a higher proportion than thestoichiometric composition can efficiently move to the semiconductorfilm 205 and oxygen vacancies in the semiconductor film 205 can becompensated. As a result, the amount of hydrogen entering thesemiconductor film 205 can be reduced, and oxygen vacancies in thesemiconductor film 205 can be reduced. Consequently, a negative shift inthreshold voltage of the transistor can be reduced, and leakage currentbetween a source and a drain of the transistor can be reduced;accordingly, the electrical characteristics of the transistor can beimproved.

In one embodiment of the present invention, as the insulating film 210,a 50-nm-thick silicon oxynitride film is formed by plasma-enhanced CVDunder the following conditions: silane with a flow rate of 20 sccm anddinitrogen monoxide with a flow rate of 3000 sccm are used as the sourcegases, the pressure in the treatment chamber is 40 Pa, the substratetemperature is 220° C., and a high-frequency power of 100 W is suppliedto parallel plate electrodes with a high-frequency power supply of 27.12MHz. Note that a plasma-enhanced CVD apparatus is a parallel plateplasma-enhanced CVD apparatus in which the electrode area is 6000 cm²,and power per unit area (power density) into which supplied power isconverted is 1.6×10⁻² W/cm². Under the above conditions, a siliconoxynitride film that passes oxygen can be formed.

As the insulating film 211, a silicon oxide film or a silicon oxynitridefilm is formed under the following conditions: the substrate placed in atreatment chamber of the plasma-enhanced CVD apparatus that isvacuum-evacuated is held at 180 to 260° C., preferably 180 to 230° C.,the pressure is 100 to 250 Pa, preferably 100 to 200 Pa withintroduction of a source gas into the treatment chamber, and ahigh-frequency power of 0.17 to 0.5 W/cm², preferably 0.25 to 0.35 W/cm²is supplied to an electrode provided in the treatment chamber.

As the deposition conditions of the insulating film 211, thehigh-frequency power having the power density is supplied to thetreatment chamber having the pressure, so that the degradationefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted. Thus, the oxygencontent in the insulating film 211 becomes higher than that in thestoichiometric composition. However, in the case where the substratetemperature is within the temperature range, a bond between silicon andoxygen is weak; thus, part of oxygen is released by heating.Accordingly, it is possible to form an oxide insulating film whichcontains oxygen at a higher proportion than the stoichiometriccomposition and from which part of oxygen is released by heating.Further, the insulating film 210 is provided over the oxide film 209.Accordingly, in the process of forming the insulating film 211, theinsulating film 210 serves as a protective film of the oxide film 209.Consequently, the insulating film 211 can be formed using thehigh-frequency power having high power density while damage to the oxidefilm 209 is reduced.

In one embodiment of the present invention, as the insulating film 211,a 400-nm-thick silicon oxynitride film is formed by plasma-enhanced CVDunder the following conditions: silane with a flow rate of 160 sccm anddinitrogen monoxide with a flow rate of 4000 sccm are used as the sourcegas, the pressure in the treatment chamber is 200 Pa, the substratetemperature is 220° C., and a high-frequency power of 1500 W is suppliedto parallel plate electrodes with a high-frequency power supply of 27.12MHz. Note that a plasma-enhanced CVD apparatus is a parallel plateplasma-enhanced CVD apparatus in which the electrode area is 6000 cm²,and power per unit area (power density) into which supplied power isconverted is 2.5×10⁻¹ W/cm².

Then, it is preferable that heat treatment be performed at least afterformation of the insulating film 211 so that oxygen contained in theinsulating film 210 or the insulating film 211 moves to the oxide film209 and the semiconductor film 205 to fill oxygen vacancies in the oxidefilm 209 and the semiconductor film 205. Note that the heat treatmentcan be performed as heat treatment for dehydration or dehydrogenation ofthe semiconductor film 205.

<Circuit Structure Examples of Semiconductor Device According to OneEmbodiment of the Present Invention>

Next, structure examples of circuits included in a semiconductor deviceaccording to one embodiment of the present invention are described.FIGS. 12A to 12C illustrate structure examples of a sequential logiccircuit 80 and a shift register 300 including the sequential logiccircuit 80.

The shift register 300 illustrated in FIG. 12A includes a plurality ofsequential logic circuits 80 (a first sequential logic circuit 80 _(—1)to an N-th sequential logic circuit 80 _(—N)) and wirings 81 to 84having a function of transmitting clock signals CLK. A clock signal CLK1is input to the wiring 81. A clock signal CLK2 is input to the wiring82. A clock signal CLK3 is input to the wiring 83. A clock signal CLK4is input to the wiring 84.

A clock signal is a signal that alternates between a high-levelpotential (H) and a low-level potential (L) at regular intervals. InFIG. 12A, the clock signals CLK1 to CLK4 are delayed by ¼ periodsequentially. In the circuits illustrated in FIGS. 12A to 12C, the clocksignals are utilized to control the sequential logic circuits 80. Notethat clock signals may also be input to the sequential logic circuits80.

The first sequential logic circuit 80 _(—1) to the N-th sequential logiccircuit 80 _(—N) each include a terminal 91, a terminal 92, a terminal93, a terminal 94, a terminal 95, a terminal 96, and a terminal 97 (seeFIG. 12B).

The terminal 91, the terminal 92, and the terminal 93 are connected toany of the wirings 81 to 84. For example, in the first sequential logiccircuit 80 _(—1), the terminal 91 is connected to the wiring 81, theterminal 92 is connected to the wiring 82, and the terminal 93 isconnected to the wiring 83. In the second sequential logic circuit 80_(—2), the terminal 91 is connected to the wiring 82, the terminal 92 isconnected to the wiring 83, and the terminal 93 is connected to thewiring 84. Note that although FIG. 12A illustrates the case wherewirings connected to the N-th sequential logic circuit 80 _(—N) are thewiring 82, the wiring 83, and the wiring 84, the wirings connected tothe N-th sequential logic circuit 80 _(—N) vary depending on the valueof N.

In the k-th sequential logic circuit (k is a natural number of 3 or moreand N or less) of the shift register 300 in one embodiment of thepresent invention, the terminal 94 is connected to the terminal 96 ofthe (k−1)th sequential logic circuit, the terminal 95 is connected tothe terminal 96 of the (k+2)th sequential logic circuit, the terminal 96is connected to the terminal 94 of the (k+1)th sequential logic circuitand the terminal 95 of the (k−2)th sequential logic circuit, and theterminal 97 outputs signals to OUT_k.

In addition, a start pulse SP1 is input from a wiring 85 to the terminal94 in the first sequential logic circuit 80 _(—1). A start pulse SP2 isinput to the terminal 95 in the (N−1)th sequential logic circuit 80_(—(N-1)). A start pulse SP3 is input to the terminal 95 in the N-thsequential logic circuit 80 _(—N). Note that the start pulse SP2 and thestart pulse SP3 may be input from the outside or generated inside thecircuit.

Next, specific structures of the first sequential logic circuit 80 _(—1)to the N-th sequential logic circuit 80 _(—N) are described.

Each of the first sequential logic circuit 80 _(—1) to the N-thsequential logic circuit 80 _(—N) includes transistors 301 to 311, asillustrated in FIG. 12C. Note that in the following description, a gateof a transistor, one of a source and a drain, and the other of thesource and the drain are referred to as a gate terminal, a firstterminal, and a second terminal, respectively.

Note that in this specification, the term “connection” means electricalconnection and corresponds to a state where current, voltage, or apotential can be supplied or transmitted. Accordingly, a connectionstate does not always mean a direct connection state but includes anindirect connection state through a circuit element such as a wiring, aresistor, a diode, or a transistor so that current, voltage, or apotential can be supplied or transmitted. Even when independentcomponents are connected to each other in a circuit diagram, there isthe case where one conductive film has functions of a plurality ofcomponents, such as the case where part of a wiring functions as anelectrode. The term “connection” in this specification also means such acase where one conductive film has functions of a plurality ofcomponents.

A source of a transistor means a source region that is part of asemiconductor film or a source electrode that is connected to thesemiconductor film. Similarly, a drain of a transistor means a drainregion that is part of the semiconductor film or a drain electrode thatis connected to the semiconductor film. A gate means a gate electrode.

The terms “source” and “drain” of a transistor interchange with eachother depending on the polarity of the transistor or levels ofpotentials applied to terminals. In general, in an n-channel transistor,a terminal to which a low potential is applied is called a source, and aterminal to which a high potential is applied is called a drain.Further, in a p-channel transistor, a terminal to which a low potentialis applied is called a drain, and a terminal to which a high potentialis applied is called a source. In this specification, although theconnection relation of the transistor is described assuming that thesource and the drain are fixed in some cases for convenience, actually,the names of the source and the drain interchange with each otherdepending on the relation of the potentials.

The structure of the sequential logic circuit illustrated in FIG. 12C isdescribed.

A first terminal of the transistor 301 is connected to the terminal 91,a second terminal of the transistor 301 is connected to the terminal 96,and a gate terminal of the transistor 301 is connected to a secondterminal of the transistor 307. A first terminal of the transistor 302is connected to the terminal 96, a second terminal of the transistor 302is connected to a wiring 71, and a gate terminal of the transistor 302is connected to a second terminal of the transistor 308. A firstterminal of the transistor 303 is connected to the terminal 91, a secondterminal of the transistor 303 is connected to the terminal 97, and agate terminal of the transistor 303 is connected to the second terminalof the transistor 307. A first terminal of the transistor 304 isconnected to the terminal 97, a second terminal of the transistor 304 isconnected to the wiring 71, and a gate terminal of the transistor 304 isconnected to the second terminal of the transistor 308. A first terminalof the transistor 305 is connected to a wiring 72, a second terminal ofthe transistor 305 is connected to a first terminal of the transistor306 and a first terminal of the transistor 307, and a gate terminal ofthe transistor 305 is connected to the terminal 94. The first terminalof the transistor 306 is connected to the second terminal of thetransistor 305 and the first terminal of the transistor 307, a secondterminal of the transistor 306 is connected to the wiring 71, and a gateterminal of the transistor 306 is connected to the second terminal ofthe transistor 308. The first terminal of the transistor 307 isconnected to the second terminal of the transistor 305 and the firstterminal of the transistor 306, the second terminal of the transistor307 is connected to the gate terminal of the transistor 301 and the gateterminal of the transistor 303, and a gate terminal of the transistor307 is connected to the wiring 72. A first terminal of the transistor308 is connected to a second terminal of the transistor 310, the secondterminal of the transistor 308 is connected to the gate terminal of thetransistor 302, the gate terminal of the transistor 304, and the gateterminal of the transistor 306, and a gate terminal of the transistor308 is connected to the terminal 92. A first terminal of the transistor309 is connected to the second terminal of the transistor 308, a secondterminal of the transistor 309 is connected to the wiring 71, and a gateterminal of the transistor 309 is connected to the terminal 94. A firstterminal of the transistor 310 is connected to the wiring 72, the secondterminal of the transistor 310 is connected to the first terminal of thetransistor 308, and a gate terminal of the transistor 310 is connectedto the terminal 93. A first terminal of the transistor 311 is connectedto the wiring 72, a second terminal of the transistor 311 is connectedto the second terminal of the transistor 308, and a gate terminal of thetransistor 311 is connected to the terminal 95.

One embodiment of the present invention is not limited to the abovestructure of the sequential logic circuit that is just an example.

In the case where the sequential logic circuit 80 in FIG. 12C is thefirst sequential logic circuit 80 _(—1) in FIG. 12A, the clock signalCLK1, the clock signal CLK2, the clock signal CLK3, the start pulse SP1,and an output signal (SROUT_3) of the third sequential logic circuit 80_(—3) are input to the terminal 91, the terminal 92, the terminal 93,the terminal 94, and the terminal 95, respectively. An output signal(SROUT_1) of the first sequential logic circuit 80 _(—1) is output fromthe terminal 96 to the terminal 94 of the second sequential logiccircuit 80 _(—2), and an output signal OUT_(—1) is output from theterminal 97.

A second potential VSS is applied to the wiring 71, and a firstpotential VDD is applied to the wiring 72.

In the shift register 300 including the sequential logic circuits 80 inFIG. 12C, desired pulses can be sequentially obtained as output signalsOUT_1 to OUT_N in response to the first potential VDD, the secondpotential VSS, the clock signals CLK1 to CLK4, the start pulse SP, andoutput signals SROUT_1 to SROUT_N.

In the case of a circuit including transistors having the sameconductivity type as illustrated in the sequential logic circuit 80 inFIG. 12C, the potentials of nodes and terminals of the circuit arelowered by the threshold voltage of the transistor. Specifically, inFIG. 12C, when the transistor 303 is on, a potential lower than ahigh-level potential (H) of a clock signal input to the terminal 91 bythe threshold voltage of the transistor 303 is applied to the terminal97. Thus, in the case of the circuit including transistors having thesame conductivity type, it is important to lower the threshold voltageof a transistor so that the transistor can be normally off.

In one embodiment of the present invention, each of the transistor 10,the transistor 30, the transistor 40, and the transistor 100 has aninitial value of threshold voltage such that each transistor is normallyoff, and the amount of change in threshold voltage in a positivedirection can be reduced. Accordingly, by using the transistor 10, thetransistor 30, the transistor 40, or the transistor 100 as each of thetransistors 301 to 311, the reliability of the sequential logic circuit80 can be increased.

In particular, in the case where the transistors 301 to 311 aren-channel transistors, a malfunction is likely to occur when thethreshold voltages of the transistor 303 for inputting a high-leveloutput signal to the terminal 97, the transistor 301 for inputting ahigh-level output signal to the terminal 96, and the transistor 305 forapplying a high-level potential to the gate terminal of each of thetransistor 303 and the transistor 301 are greatly shifted in a positivedirection. For example, the sequential logic circuit 80 does not operatecorrectly or high-level potentials output from the terminal 96 and theterminal 97 become lower than desired potentials even when thesequential logic circuit 80 operates correctly. Accordingly, the use ofthe transistor 10, the transistor 30, the transistor 40, or thetransistor 100 as at least each of the transistor 303, the transistor301, and the transistor 305 is effective in ensuring the reliability ofthe sequential logic circuit 80.

Note that in one embodiment of the present invention, in the structureof the sequential logic circuit illustrated in FIG. 12C, a back gate isprovided for each of the transistors. The back gate may be floating ormay be supplied with a potential from another element. In the lattercase, potentials at the same level may be applied to a normal gate(front gate) and the back gate, or a fixed potential such as a groundpotential may be applied only to the back gate. By controlling thepotential applied to the back gate, the threshold voltage of thetransistor can be controlled. By providing the back gate, a channelformation region is enlarged and drain current can be increased.Further, the back gate facilitates formation of a depletion layer in thesemiconductor film, which results in lower subthreshold swing.

<Structure Example of Semiconductor Display Device>

In one embodiment of the present invention, a structure example of asemiconductor display device that is one aspect of a semiconductordevice of the present invention is described.

In a panel 460 in FIG. 13A, a plurality of pixels 462, scan lines GL(GL1 to GLm (m is a natural number)) for selecting the pixels 462 row byrow, and signal lines SL (SL1 to SLn (n is a natural number)) forsupplying image signals to the selected pixels 462 are provided in apixel portion 461. Input of signals to the scan lines GL is controlledby a scan line driver circuit 463. Input of image signals to the signallines SL is controlled by a signal line driver circuit 464. Each of theplurality of pixels 462 is connected to at least one of the scan linesGL and at least one of the signal lines SL.

Note that the kinds and number of the lines in the pixel portion 461 canbe determined by the structure, number, and position of the pixels 462.Specifically, in the case of the pixel portion 461 in FIG. 13A, thepixels 462 are arranged in a matrix of n columns×m rows, and the signallines SL1 to SLn and the scan lines GL1 to GLm are provided in the pixelportion 461.

The sequential logic circuit 80 and the shift register 300 illustratedin FIGS. 12A to 12C can be used for the scan line driver circuit 463 orthe signal line driver circuit 464. The use of the sequential logiccircuit 80 and the shift register 300 including the transistor 10, thetransistor 30, the transistor 40, or the transistor 100 for the scanline driver circuit 463 or the signal line driver circuit 464 increasesthe reliability of the semiconductor display device.

FIG. 13B illustrates a structure example of the pixel 462. Each of thepixels 462 includes a liquid crystal element 465, a transistor 466controlling supply of an image signal to the liquid crystal element 465,and a capacitor 467 for holding voltage across a pixel electrode and acommon electrode of the liquid crystal element 465. The liquid crystalelement 465 includes the pixel electrode, the common electrode, and aliquid crystal layer that is provided between the pixel electrode andthe common electrode, is supplied with voltage, and contains a liquidcrystal material.

The transistor 466 controls whether to apply the potential of the signalline SL to the pixel electrode of the liquid crystal element 465. Apredetermined potential is applied to the common electrode of the liquidcrystal element 465.

The specific connection relation between the transistor 466 and theliquid crystal element 465 is described below. In FIG. 13B, a gateelectrode of the transistor 466 is connected to any one of the scanlines GL1 to GLm. One of a source electrode and a drain electrode of thetransistor 466 is connected to any one of the signal lines SL1 to SLn.The other of the source electrode and the drain electrode of thetransistor 466 is connected to the pixel electrode of the liquid crystalelement 465.

In FIG. 13B, one transistor 466 is used in the pixel 462 as a switchcontrolling input of an image signal to the pixel 462; however, aplurality of transistors functioning as one switch may be used in thepixel 462.

In one embodiment of the present invention, the use of the transistor10, the transistor 30, the transistor 40, or the transistor 100 as thetransistor 466 increases the reliability of the semiconductor displaydevice. A transistor including an oxide semiconductor in a semiconductorfilm has extremely low off-state current; thus, when such a transistoris used as the transistor 466, leakage of electric charge through thetransistor 466 can be prevented. Thus, the potential of an image signalthat is applied to the liquid crystal element 465 and the capacitor 467can be held more reliably. Accordingly, changes in transmittance of theliquid crystal element 465 due to leakage of electric charge in oneframe period are prevented, so that the quality of an image to bedisplayed can be improved. In addition, when the off-state current ofthe transistor 466 is low, leakage of electric charge through thetransistor 466 can be prevented; thus, the area of the capacitor 467 canbe made small. Consequently, the transmittance of the panel 460 isincreased, so that loss of light supplied from a light supply portionsuch as a backlight or a frontlight in the panel 460 and the powerconsumption of a liquid crystal display device can be reduced.Alternatively, in a period during which a still image is displayed,supply of power supply potentials or signals to the scan line drivercircuit 463 and the signal line driver circuit 464 may be stopped. Withsuch a structure, the number of times of writing image signals to thepixel portion 461 can be decreased, so that the power consumption of thesemiconductor display device can be reduced.

FIG. 13B illustrates another example of the pixel 462. The pixel 462includes a transistor 470 controlling input of an image signal to thepixel 462, a light-emitting element 473, a transistor 471 controllingthe value of current supplied to the light-emitting element 473 inresponse to an image signal, and a capacitor 472 for holding thepotential of an image signal.

The potential of one of an anode and a cathode of the light-emittingelement 473 is controlled in response to an image signal input to thepixel 462. A predetermined potential is applied to the other of theanode and the cathode of the light-emitting element 473. The luminanceof the light-emitting element 473 is determined by a potentialdifference between the anode and the cathode. In each of the pluralityof pixels 462 included in the pixel portion, the luminance of thelight-emitting element 473 is adjusted in response to an image signalcontaining image information, so that an image is displayed on the pixelportion 461.

Next, connection of the transistor 470, the transistor 471, thecapacitor 472, and the light-emitting element 473 that are included inthe pixel 462 is described.

One of a source electrode and a drain electrode of the transistor 470 isconnected to the signal line SL, and the other of the source electrodeand the drain electrode of the transistor 470 is connected to a gateelectrode of the transistor 471. A gate electrode of the transistor 470is connected to the scan line GL. One of a source electrode and a drainelectrode of the transistor 471 is connected to a power supply line VL,and the other of the source electrode and the drain electrode of thetransistor 471 is connected to the light-emitting element 473.Specifically, the other of the source electrode and the drain electrodeof the transistor 471 is connected to one of the anode and the cathodeof the light-emitting element 473. A predetermined potential is appliedto the other of the anode and the cathode of the light-emitting element473.

Note that in FIG. 13C, the pixel 462 includes the capacitor 472.However, for example, in the case where gate capacitance generatedbetween the gate electrode and a semiconductor film of the transistor470 or gate parasitic capacitance is high, i.e., the case where thepotential of an image signal can be sufficiently held by anothercapacitor, the capacitor 472 is not necessarily provided in the pixel462.

Examples of the light-emitting element 473 include an element whoseluminance is controlled by current or voltage, such as a light-emittingdiode (LED) or an organic light-emitting diode (OLED). For example, anOLED includes at least an EL layer, an anode, and a cathode. The ELlayer is formed using a single layer or a plurality of layers providedbetween the anode and the cathode, at least one of which is alight-emitting layer containing a light-emitting substance.

From the EL layer, electroluminescence is obtained by current suppliedwhen a potential difference between the cathode and the anode is higherthan or equal to the threshold voltage of the light-emitting element473. Electroluminescence includes luminescence (fluorescence) at thetime of returning from a singlet-excited state to a ground state andluminescence (phosphorescence) at the time of returning from atriplet-excited state to a ground state.

<Structure Example of Electronic Device Including Semiconductor Device>

A semiconductor device according to one embodiment of the presentinvention can be used for display devices, personal computers, or imagereproducing devices provided with recording media (typically, devicesthat reproduce the content of recording media such as digital versatilediscs (DVDs) and have displays for displaying the reproduced images).Further, as electronic devices that can include the semiconductor deviceaccording to one embodiment of the present invention, cellular phones,game machines (including portable game machines), portable informationterminals, e-book readers, cameras such as video cameras and digitalstill cameras, goggle-type displays (head mounted displays), navigationsystems, audio reproducing devices (e.g., car audio systems and digitalaudio players), copiers, facsimiles, printers, multifunction printers,automated teller machines (ATMs), vending machines, and the like can begiven. FIGS. 14A to 14F illustrate specific examples of these electronicdevices.

FIG. 14A illustrates a portable game machine, which includes a housing5001, a housing 5002, a display portion 5003, a display portion 5004, amicrophone 5005, speakers 5006, an operation key 5007, a stylus 5008,and the like. It is possible to use the semiconductor device accordingto one embodiment of the present invention as the display portion 5003or 5004 or another circuit. Note that although the portable game machinein FIG. 14A has the two display portions 5003 and 5004, the number ofdisplay portions included in the portable game machine is not limitedthereto.

FIG. 14B illustrates a display device, which includes a housing 5201, adisplay portion 5202, a support 5203, and the like. It is possible touse the semiconductor device according to one embodiment of the presentinvention as the display portion 5202 or another circuit. Note that thedisplay device means all display devices for displaying information,such as display devices for personal computers, for receiving TVbroadcast, and for displaying advertisements.

FIG. 14C illustrates a laptop, which includes a housing 5401, a displayportion 5402, a keyboard 5403, a pointing device 5404, and the like. Itis possible to use the semiconductor device according to one embodimentof the present invention as the display portion 5402 or another circuit.

FIG. 14D illustrates a portable information terminal, which includes afirst housing 5601, a second housing 5602, a first display portion 5603,a second display portion 5604, a joint 5605, an operation key 5606, andthe like. The first display portion 5603 is provided in the firsthousing 5601, and the second display portion 5604 is provided in thesecond housing 5602. The first housing 5601 and the second housing 5602are connected to each other with the joint 5605, and an angle betweenthe first housing 5601 and the second housing 5602 can be changed withthe joint 5605. An image on the first display portion 5603 may beswitched depending on the angle between the first housing 5601 and thesecond housing 5602 at the joint 5605. It is possible to use thesemiconductor device according to one embodiment of the presentinvention as the first display portion 5603, the second display portion5604, or another circuit. A semiconductor device with a position inputfunction may be used as at least one of the first display portion 5603and the second display portion 5604. Note that the position inputfunction can be added by providing a touch panel in a semiconductordevice. Alternatively, the position input function can be added byproviding a photoelectric conversion element called a photosensor in apixel portion of a semiconductor device.

FIG. 14E illustrates a video camera, which includes a first housing5801, a second housing 5802, a display portion 5803, operation keys5804, a lens 5805, a joint 5806, and the like. The operation keys 5804and the lens 5805 are provided in the first housing 5801, and thedisplay portion 5803 is provided in the second housing 5802. The firsthousing 5801 and the second housing 5802 are connected to each otherwith the joint 5806, and an angle between the first housing 5801 and thesecond housing 5802 can be changed with the joint 5806. An image on thedisplay portion 5803 may be switched depending on the angle between thefirst housing 5801 and the second housing 5802 at the joint 5806. It ispossible to use the semiconductor device according to one embodiment ofthe present invention as the display portion 5803 or another circuit.

FIG. 14F illustrates a cellular phone, which includes a display portion5902, a microphone 5907, a speaker 5904, a camera 5903, an externalconnection port 5906, and an operation button 5905 in a housing 5901. Itis possible to use the semiconductor device according to one embodimentof the present invention as a circuit of the cellular phone. When aliquid crystal display device that is one of semiconductor devicesaccording to one embodiment of the present invention is provided over aflexible substrate, the liquid crystal display device can be used as thedisplay portion 5902 having a curved surface, as illustrated in FIG.14F.

This application is based on Japanese Patent Application serial No.2012-251935 filed with Japan Patent Office on Nov. 16, 2012, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a gateelectrode; a gate insulating film; a first oxide semiconductor filmoverlapping with the gate electrode with the gate insulating filmpositioned between the first oxide semiconductor film and the gateelectrode; a second oxide semiconductor film over the first oxidesemiconductor film; a third oxide semiconductor film over the secondoxide semiconductor film; and a source electrode and a drain electrodebeing in contact with the third oxide semiconductor film and the gateinsulating film, wherein an end portion of the second oxidesemiconductor film is spaced from an end portion of the source electrodeor the drain electrode in a region overlapping with the second oxidesemiconductor film in a channel width direction, wherein a width of thesecond oxide semiconductor film in the channel width direction is largerthan a width of the source electrode or the drain electrode in thechannel width direction, and wherein an energy level of a bottom of aconductive band of the second oxide semiconductor film is lower than anenergy level of a bottom of a conductive band of the first oxidesemiconductor film and an energy level of a bottom of a conductive bandof the third oxide semiconductor film.
 2. The semiconductor deviceaccording to claim 1, wherein the second oxide semiconductor filmcomprises In, Ga, and Zn.
 3. The semiconductor device according to claim1, wherein directions of a-axes and b-axes are different between crystalparts in the second oxide semiconductor film, and wherein c-axes arealigned in a direction parallel to a normal vector of a formationsurface or a normal vector of a top surface.
 4. The semiconductor deviceaccording to claim 1, further comprising: a substrate, wherein the gateelectrode is over the substrate.
 5. An electronic device comprising thesemiconductor device according to claim
 1. 6. The semiconductor deviceaccording to claim 1, wherein the energy level of the bottom of theconductive band of the first oxide semiconductor film is continuouslychanged to the energy level of the bottom of the conductive band of thesecond oxide semiconductor film at an interface between the second oxidesemiconductor film and the first oxide semiconductor film, and whereinthe energy level of the bottom of the conductive band of the third oxidesemiconductor film is continuously changed to the energy level of thebottom of the conductive band of the second oxide semiconductor film atan interface between the third oxide semiconductor film and the secondoxide semiconductor film.
 7. A semiconductor device comprising: a gateelectrode; a gate insulating film over the gate electrode; a first oxidesemiconductor film overlapping with the gate electrode with the gateinsulating film positioned between the first oxide semiconductor filmand the gate electrode; a source electrode and a drain electrode beingin contact with the first oxide semiconductor film; and an oxide filmover the first oxide semiconductor film, the source electrode, and thedrain electrode, wherein an end portion of the first oxide semiconductorfilm is spaced from an end portion of the source electrode or the drainelectrode in a region overlapping with the first oxide semiconductorfilm in a channel width direction, wherein a width of the first oxidesemiconductor film in the channel width direction is larger than a widthof the source electrode or the drain electrode in the channel widthdirection, wherein the first oxide semiconductor film and the oxide filmeach comprise a metal oxide comprising In, Ga, and Zn, and wherein theoxide film has an atomic ratio where an atomic percent of In is lowerthan an atomic percent of In in an atomic ratio of the first oxidesemiconductor film.
 8. The semiconductor device according to claim 7,wherein directions of a-axes and b-axes are different between crystalparts in the first oxide semiconductor film, and wherein c-axes arealigned in a direction parallel to a normal vector of a formationsurface or a normal vector of a top surface.
 9. The semiconductor deviceaccording to claim 7, further comprising: a substrate, wherein the gateelectrode is over the substrate.
 10. An electronic device comprising thesemiconductor device according to claim
 7. 11. The semiconductor deviceaccording to claim 7, further comprising: a second oxide semiconductorfilm being in contact with the first oxide semiconductor film; and athird oxide semiconductor film being in contact with the first oxidesemiconductor film, wherein the first oxide semiconductor film isbetween the second oxide semiconductor film and the third oxidesemiconductor film, and wherein an energy level of a bottom of aconductive band of the first oxide semiconductor film is lower than anenergy level of a bottom of a conductive band of the second oxidesemiconductor film and an energy level of a bottom of a conductive bandof the third oxide semiconductor film.
 12. A semiconductor devicecomprising: a gate electrode; a gate insulating film; a semiconductorfilm overlapping with the gate electrode with the gate insulating filmpositioned between the semiconductor film and the gate electrode; and asource electrode and a drain electrode, wherein each of the sourceelectrode and the drain electrode comprises a plurality of convexportions in an end portion, wherein the plurality of convex portions areeach partly in contact with the semiconductor film, wherein an endportion of the semiconductor film is spaced from the plurality of convexportions in a region overlapping with the semiconductor film in achannel width direction, and wherein the semiconductor film comprises anoxide semiconductor.
 13. The semiconductor device according to claim 12,wherein the semiconductor film comprises In, Ga, and Zn.
 14. Thesemiconductor device according to claim 12, further comprising: an oxidefilm over the semiconductor film, the source electrode, and the drainelectrode, wherein the oxide film has an atomic ratio where an atomicpercent of In is lower than an atomic percent of In in an atomic ratioof the semiconductor film.
 15. The semiconductor device according toclaim 12, wherein directions of a-axes and b-axes are different betweencrystal parts in the semiconductor film, and wherein c-axes are alignedin a direction parallel to a normal vector of a formation surface or anormal vector of a top surface.
 16. The semiconductor device accordingto claim 12, further comprising: a substrate, wherein the gate electrodeis over the substrate.
 17. An electronic device comprising thesemiconductor device according to claim
 12. 18. The semiconductor deviceaccording to claim 12 wherein the source electrode and the drainelectrode being in contact with the gate insulating film.
 19. Thesemiconductor device according to claim 11, wherein the energy level ofthe bottom of the conductive band of the first oxide semiconductor filmis continuously changed to the energy level of the bottom of theconductive band of the second oxide semiconductor film at an interfacebetween the second oxide semiconductor film and the first oxidesemiconductor film, and wherein the energy level of the bottom of theconductive band of the third oxide semiconductor film is continuouslychanged to the energy level of the bottom of the conductive band of thefirst oxide semiconductor film at an interface between the third oxidesemiconductor film and the first oxide semiconductor film.